A servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism. The term correctly applies only to systems where the feedback or error-correction signals help control mechanical position or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation. By contrast the car’s cruise control uses closed loop feedback, which classifies it as a servomechanism.
A servomechanism may or may not use a servomotor. For example, a household furnace controlled by a thermostat is a servomechanism, yet there is no motor being controlled directly by the servomechanism. A common type of servo provides position control.
Introduction to Servo Amplifiers
The output stage of all servo amplifiers is an analog circuit. The analog circuit provides a means to allow the voltage and current for the motor to be adjusted to control position, velocity, and torque. The feedback and comparator stages can be any mixture of digital and analog devices. For example, if the feedback section uses a resolver, the output of this device is analog, so the section it works with is generally also analog. If the feedback device is an encoder, its output is digital, and the digital signal can be converted through a frequency-to-voltage converter so that the signal is usable in an analog circuit. Or it can be filtered and can use a digital value. The advent of microprocessors has allowed the digital values to be used through every part of the servo controller except the final output stage.
Figure 1 shows a diagram of the components in a typical servo linear amplifier. The circuit shows the motor winding connected to a set of transistors (TR1 and TR2). The transistors can control positive (+V) or negative (—V) voltage to make the motor turn in the clockwise or counterclockwise direction. The transistors can be pulsed on and off as in a pulse-width modulation (PWM) circuit, or they can ramped up and down as a simple linear circuit. The base of the transistors can be controlled by a controller section of an amplifier that is completely linear. Or the controller can be digital with a D/A converter to provide the analog control signal to the base of Q1 and Q2, or the controller can be completely digital and the base of transistors Q1 and Q2 can be pulsed directly by the digital controller. Typically IGBTs (insulated gate bipolar transistors) are used in modern servo amplifiers where PWM or other switching circuits are used. The IGBTs allow the transistors to be switched on and off at frequencies that limit the harmonic hum in a motor or amplifier. The high-pitched hum represents both audio and electronic noise that must be eliminated or controlled.
A typical linear amplifier for a servo system.
The amplifier circuits for DC servomotors are similar to the AC circuits used for pulse-width modulation or other switching systems. In fact the complete amplifier for AC servomotors will be similar to the variable-frequency drive amplifiers shown earlier in this chapter.
Early Amplifiers (Push-Pull Amps)
The design of amplifiers has changed rapidly over the last 15 years because transistors, triacs, and SCRs have become able to handle larger voltages and currents without damaging themselves. It is easy to see the advantages that the changes in these devices have brought to motor drive amplifiers, but you must keep in mind that the early amplifiers were built so well that you will run into them even today when you are asked to troubleshoot a drive system. For this reason it is prudent to leam their basic parts and functions so you will be able to troubleshoot and analyze them. It is also a good idea to understand their basic operation because this is what has been modified to make the newer drives more efficient and more powerful.
One of the earliest types of linear amplifiers is called a push-pull amplifier, which was designed so that two transistors switched on and off to share the current load for the motor. Figure 2 shows an example of this type of amplifier, and you can see that Q2and Q3 are the power transistors. They are connected to the primary winding of transformer T2. The servomotor winding is connected to the secondary winding of transformer T2.
The operation of the push-pull amplifier begins with a sine wave signal that enters the input of the push-pull amplifier through capacitor C1. Capacitor C1 makes sure that the input signal is a pure sine wave with no DC bias. The base circuit of transistor Q1 has a DC bias on it of approximately 13.5 volts. The base-emitter junction needs only 0.7 volt to turn it on, the rest of the DC bias voltage providing DC current through R2 and R3. This causes the sine-wave input signal to practically turn transistor Q1 off at its minimum, causing Q2 to be driven almost into saturation at the sine wave’s maximum. This causes current to flow through the primary winding of transformer T1. Notice the secondary of T1 is center tapped to ground. When the positive half of the sine wave appears on the secondary, it appears across the entire secondary. Because of the center tap, only the upper portion of the secondary sees a positive voltage, and this forward bias transistor Q2 allows it to conduct. Transistor Q3 is turned off because it sees a negative voltage at its base. With Q2 conducting, current flows up through the primary of T2, providing a positive pulse to the secondary of T2. When Q3 conducts, a negative pulse is provided to the primary of T2.
Another type of early amplifier for a servomotor is called the chopper amplifier (see Fig. 3). In this type of amplifier the positive rectangular DC pulses arrive at the input of the amplifier circuit at capacitor C1. These pulses arrive at the base of Q1 as narrow spikes, which momentarily turn Q1on. This in turn momentarily turns Q2 on, which allows current to flow through the primary of transformer T1. Now the primary of transformer T1 is really an L-C tank circuit. (Remember that the primary winding of the transformer is actually a big inductor.) When this tank circuit is hit by a pulse, it will produce a cycle or two of pure sine wave. When hit, in other words, the tank circuit will ring like a bell. The amplifier circuit is the clapper that rings the bell. Notice the secondary of T1 is center tapped to —60 Vp. The secondary of T1 sees a pure AC sine wave, and to this AC signal, the —60Vp appears as a ground. This means that for the positive half-cycle of the sine wave, Q3 would see a positive pulse, and Q4 would see a negative pulse. Both power transistors are NPN transistors, so a positive bias is needed at the base to cause them to conduct. As both bases are grounded , Q4 would go into conduction because its emitter is lower than its base, giving it a forward base-emitter bias. The output of the tapped control winding would then be a sine wave. It should be noticed that the tapped control winding has +60 Vp on it, and the secondary of T1 has —60 Vp on it. This means that the output of the tapped control winding is going to be a 120-Vp sine wave.
FIGURE 3 Output stages of a chopper amplifier for an AC servomotor.
FIGURE 4 Two-transistor amplifier for a DC servomotor.